Related Art
Heretofore, a vacuum assistor apparatus (also termed an negative pressure servo apparatus) has been adopted in a vehicle as a brake booster for reducing the operating force during braking.
The vacuum servo apparatus, in which the negative suction pressure of an engine or an negative pressure of a vacuum pump is used as a servo force and, using the pressure difference between this negative pressure and the atmospheric pressure, a force exceeding the foot pressure of a driver applied to a braking pedal is hydraulically applied to a braking device, is a most commonplace system as a brake booster.
In, for example, a German Patent DE 4405092C1, a vacuum servo apparatus having a mechanism producing "additional jumping", in which, if a brake pedal actuating speed by a vehicle driver exceeds a threshold value, e.g., during rapid braking, a solenoid in a servo apparatus is excited for adding and generating an output other than the biasing force by a driver's input, in association with the driving or braking conditions, has been proposed.
Meanwhile, the "additional jumping" means a phenomenon in which an output value in terms of an output/input ratio of a vacuum servo is rapidly increased in the absence of the input increase.
FIG. 8 illustrates a prior art and is a partial cross-sectional view of a vacuum servo apparatus proposed in the above German Patent DE 4405092C1, taken along the input/output shaft of the vacuum servo apparatus.
Referring to FIG. 8, the vacuum servo apparatus includes, in addition to detection means, not shown, adapted for detecting the brake pedal pressing velocity by a driver, a power piston 622 slidable along an input/output shaft 699 under a differential pressure between the negative pressure and the atmospheric pressure, and an input rod 644 on which is applied the foot pressure by the driver. The power piston 622 and the input rod 644 are provided on the outer periphery of the apparatus and on the inner periphery of the apparatus (within the power piston 622), respectively.
Along the axial line 699, looking from the input towards the output, there are mounted a transmission member 641, mutually interacting with the input rod 644, a reaction disc 648, abutted on the inner periphery and on the outer periphery thereof against the input rod 622 and against the power piston 622, respectively, and mutually interacting with the input rod 622, and an output rod 650 abutted against the reaction disc 648 and mutually interacting with the braking device. Between the outer periphery of the transmission member 641 and the power piston 622, there is provided a solenoid coil 640 extending parallel to the axial line 699, whereas, between the outer periphery of the transmission member 641 and the inner periphery of the solenoid coil, there is arranged a solenoid plunger 630 sucked towards the output under an electromagnetic force generated by the solenoid 640. The solenoid plunger 630 has an atmospheric valve element 632.
Between the outer periphery of the input rod 644 and the inner periphery of the power piston 622, there are provided an atmospheric valve 624 and a negative pressure valve 611. The atmospheric valve 624 is made up of a valve seat 660 retained by the power piston 622 and biased by a coil spring 700 towards an output side and the above-mentioned atmospheric valve element 632 facing the inner periphery of the output side end face of the valve seat 660. The atmospheric valve element 632 is provided in the solenoid plunger 630 and is extended along the input/output shaft 699. Between the atmospheric valve element 632 and the transmission member 641 is interposed a coil spring 638. The negative pressure valve 611 is made up of the valve seat 660 and an negative pressure valve element 623 facing the outer periphery of the output side end face of the valve seat 660 and formed within the inside of the power piston 622 for encircling the outer periphery of the atmospheric valve element 632.
It is only the surface formed on the reaction disc retainer 622 for facing the forward end face of the solenoid plunger 630 that operates as a stop for restricting movement of the atmospheric element 632 towards the output side. In addition, there is no clearance between the transmission member 641 and the reaction member 648. Therefore, the opening degree of the atmospheric valve 624 is substantially equal to the initial distance between the forward end face of the solenoid plunger 630 and the face of the reaction disc retainer 622 facing the froward end face, that is a clearance shown in FIG. 6. This distance corresponds to the maximum opening degree.
The operation of the vacuum servo apparatus shown in FIG. 8 is explained by referring to FIGS. 8 and 9. In the graph of FIG. 9, showing the performance of the vacuum servo apparatus, lines a, c denote operating lines for usual braking and for the case in which the solenoid of the conventional vacuum servo apparatus is excited for supplementing and generating an output other than a biasing force imposed by the input.
If an input f.sub.1 is applied by a driver on the input rod 644, the vacuum servo apparatus generates an output a.sub.1. If, in the course of application of the input f.sub.1 is applied, the solenoid in the servo apparatus is excited, as a result of detection of the emergency braking state or for other reasons, for supplementing and generating an output other than the biasing force by the input.
The solenoid plunger 630 is moved towards the output side under the electromagnetic force generated by the solenoid coil 640 so that the forward end face of the solenoid plunger 630 is abutted against a surface formed on the reaction disc retainer 622 for facing the forward end face. The atmospheric valve element 632 provided on the solenoid plunger 630 is also moved towards the output side for opening the valve 624 to permit atmospheric air to be intruded into a variable pressure chamber for generating a force of driving the power piston 622 towards the output side.
The reaction member 648 is elastically deformed by the thrusting force exerted by the power piston 622 for generating a force of reaction for thrusting back the input member 641 so that the input member 641 and the input rod 644 is moved towards the input side. However, since the atmospheric valve 632 is not moved towards the input side, the atmospheric valve 624 keeps on to be opened, so that the pressure within the variable pressure chamber reaches the atmospheric pressure. Thus the vacuum servo apparatus is in the maximum assisting state and generates an output c.sub.1. Subsequently, the vacuum servo apparatus operates on a line c.
That is, if current is supplied through the solenoid coil 640 at an input f.sub.1 an output jumps at once to c.sub.1 from an output a.sub.1 under an additional jumping.